Abstract:
Radiolytic synthesis routes and wastewater remediation methodologies exploit the chemical effects produced by the interaction of high-energy γ-rays and electrons with liquids to produce nanomaterials and to break-down water pollutants, respectively. In the (scanning) transmission electron microscope, (S)TEM, the formation and degradation dynamics of nanomaterials can be investigated in-situ using the effect of the 60-300kV incident electrons in combination with liquid cells. Many theoretical and experimental efforts have been devoted to understanding how irradiation in the (S)TEM affects DI water and different aqueous solutions, since water is the most common solvent used in the electron microscope. Furthermore, radiation chemists have used water for decades for general kinetics studies, thus producing a large amount of information that is now available for potential EM applications. These kinetics studies provide a base to understand drastic experimental differences observed when small changes to the initial composition of the solution are made. For instance, competing kinetics can explain why the pH decreases during irradiation of DI water while it can increase when including certain solutes in the solution. More generally, radiation-chemical methods can be applied to liquid cell experiments to tune the environment for specific applications in the fields of chemistry and materials sciences.
In this seminar I will introduce experimental conditions for which the solvent in a solution dictates the radiation chemistry during in-situ liquid cell EM, which main factors change the production of radicals and the basics of competing kinetics in radiation chemistry. I will discuss the case of water more in detail but also introduce some general concepts on the radiation chemistry of organic solvents and specific properties that can benefit liquid cells experiments; such as the possibility of minimal production of reactive species or of net production of molecular species of lower reducing/oxidizing power. Finally, I will discuss general methods for finding more suitable synthesis/corrosive environments for controlled nanoparticles formation or dissolution using the electron beam by either reproducing a selective reducing or oxidizing environment and show examples in the (S)TEM when available.

Abstract:
I describe the emergence of color superfluid phases for systems of ultra-cold atoms with artificial color-orbit and color-flip fields for three-component (Red-Green-Blue) Fermi systems. For fermions interacting only in the s-wave channel, I describe the phase diagrams of color-flip fields versus interaction parameter for fixed color-orbit coupling. Various topological phases are encountered, where the quasiparticle excitation spectrum possesses nodal structures induced by the presence of color-orbit coupling. The order parameter tensor develops momentum dependent non-zero matrix elements in the singlet, triplet and quintet sectors. An unusual topological multicritical point arises, where several gapless phases converge. This point can be reached by tuning color-flip fields and interaction parameter. Finally, I discuss potential experiments to observe these phases in Fermi isotopes of Lithium, Potassium and Ytterbium.

Dinsdag 5 december 2017, 16.30 u., Lokaal N0.08 (Campus Drie Eiken)

Voordracht georganiseerd door TQC

Onderwerp: Semiclassical simulation of interacting fermions
Spreker: Dries Sels, TQC, University of Antwerp and Boston University

Abstract:
Already in equilibrium, fermions are notoriously hard to handle due to the sign problem. Out of equilibrium, an important outstanding problem is the efficient numerical simulation of the dynamics of these systems. In talk I will discuss a new semiclassical phase-space approach (a.k.a. the truncated Wigner approximation) for simulating the dynamics of interacting fermions in arbitrary dimensions. As fermions are essentially non-classical objects, a phase-space is constructed out of all fermionic bilinears. Classical phase-space is thus comprised of highly non-local (hidden) variables representing these bilinears, and the cost of the method is that it scales quadratic rather than linear with system size. We demonstrate the strength of the method by comparing the results to the exact quantum dynamics of fermion expansion in the Hubbard model and quantum thermalization in the Sachdev-Ye-Kitaev (SYK) model for small systems, where the semi-classics nearly perfectly reproduces correct results.

Abstract:
Scanning transmission electron microscopy (STEM) is a well established experimental technique for characterizing materials down to sub-Ångstrøm resolutions. It is based on scanning a convergent beam across a thin sample, giving a convergent beam electron diffraction pattern in the back focal plane. Historically, one has used detectors which integrate up large fractions of these scattered electrons into a single intensity value for each scan point. However, this discards much of the rich information contained in the back focal plane. Recently, advances in fast electron detectors has enabled the imaging of these diffraction patterns at 1000+ frames per second.
This lecture will focus on the practical aspects and applications of using such a fast pixelated detector: the Medipix3. It will cover aspects such as live processing and visualization, file formats and data processing. In addition, it will cover applications such as magnetic differential phase contrast imaging, 3D-structure mapping using higher order laue zones, and fluctuation electron microscopy of amorphous thin films.

Abstract:
In this seminar I will consider the problem of avoided crossings in phononic crystals and explain a recent computationally strategy derived by Y. Lu and A. Srivastava to distinguish them from normal cross points. This process is essential for the correct sorting of the phononic bands and, subsequently, for the accurate determination of mode continuation, group velocities, and emergent properties which depend on them such as thermal conductivity and transport coefficients.

Onderwerp: How to visualize and characterize soft-hard materials?
Spreker : Nathalie Claes, EMAT, University of Antwerp

Abstract:
Metallic nanoparticles show interesting properties which makes them widely applicable. After combination with soft materials, such as polymers or proteins, the properties of the system can be drastically improved. A decrease of aggregation, encouragement of self-assembly and counteraction of oxidation are only a few examples of the functionalities that can be tuned. However, the size, thickness, uniformity and morphology of the components in the soft-hard material will affect the properties of the system. This makes a thorough structural characterization of great importance.
Simultaneously visualizing the soft and hard material is far from straightforward due to the sensitivity of the soft material towards the electron beam. Furthermore, the contrast of the soft materials is weak, which makes the visualization and characterization more challenging. We used different 2D and 3D electron microscopy techniques to overcome these difficulties. In this talk we will discuss the different challenges and solutions we encountered in three different soft-hard systems.

Onderwerp : The conductance of edge states of two-dimensional topological insulators
Spreker: Prof. Thomas Schmidt, University of Luxemburg

Abstract:
Two-dimensional topological insulators are characterized by behaving as insulators in the bulk whilst hosting metallic edge states. In the presence of time-reversal symmetry, these edge states have been predicted to be robust against certain backscattering mechanisms, which might make them promising candidates for low-dissipation electronics devices.
Experiments in various materials have indeed found evidence for these edge states, recently even at non-cryogenic temperatures, but dissipation always seems to be significant. Hence, it remains a question whether ideal edge states, i.e., in samples free of disorder, might at least theoretically allow for dissipationless transport.
In this talk, I will present the most important backscattering mechanisms which are effective in topological insulator edge states. Interactions either among electrons or between electrons and phonons are required to produce dissipation. I will discuss the importance of the possible interaction processes in different temperature ranges and for different system lengths, and thus estimate the maximal conductivity of an ideal edge state.

Abstract:
The dependence of the excitonic absorbance spectrum of monolayer transition metal dichalcogenides (TMDs) on the tilt angle of an applied magnetic field is studied. Starting from a four-band Hamiltonian we construct a theory which quantitatively reproduces the available experimental excitonic absorbance spectra for perpendicular and in-plane magnetic fields. In the presence of a tilted magnetic field, we demonstrate that the dark exciton absorbance peaks brighten due to the in-plane component of the magnetic field and split for light with different circular polarization as a consequence of the perpendicular component of the magnetic field. In tungsten-based TMDs this splitting is more than twice as large as the splitting of the bright exciton peaks, which should be observable experimentally.

Abstract:
In my talk I will present the results of advanced electron microscopy applications to the characterization of filled single-walled carbon nanotubes (SWNTs) as one-dimensional objects and the use of a combination of (S)TEM, EBSD, EDX and image simulation in the structural study of textured thermoelectric films.
In the first part I will present structural data of Single-walled carbon nanotubes (SWNTs) filled by 1-bromoadamantane, HgCl2 and CuCl molecules. Single-walled carbon nanotubes (SWNTs) are among the most effective and universal containers for molecule incorporation. Doping of SWCNT with inorganic materials can increase the conductivity and change the optical absorption. The inner volume of SWNT could be used as a nanoscale chemical reactor, where the structure of the macromolecular product can be precisely controlled by spatial confinement of the reactions in the nanotube channel. HRTEM is an effective technique for direct observation of the local crystal chemistry of the incorporated crystals with atom-scale resolution. Properties of the nanocomposite based on filled nanotubes can be correctly interpreted only if the structural organization of internal channel of the nanotube is revealed. HRTEM analysis of filled SWNT showed that 1-bromoadamantane molecules in the inner channel of nanotube formed some carbon structures with free standing Br-atoms. The HgCl2 molecules transform to the Hg2Cl2 structure and CuCl molecules transform to CuCl2 with MoS2-type unit cell.
The second part will focus on the structural study of thermoelectric films of higher manganese silicide (HMS) crystals. Growing interest for energy-saving technologies leads to the development of new devices based on crystals with thermoelectric properties. Semiconducting silicides of transition metals have a high potential for thermoelectric applications, they are nontoxic and have low-cost of initial components. HMS has the highest value of figure of merit among the semiconducting silicides. Analysis of the chemical and the phase composition didn’t reveal the presence of any precipitates and in particular the presence of manganese monosilicide. After manganese deposition on a Si(111) substrate textured HMS films are formed. The interface of the HMS/Si-substrate has a semi-coherent structure with a net of misfit dislocations. Textured analysis revealed two preferred orientations which were analyzed by atomic modeling.

Abstract:
Metal nanorods (NRs) have unique optical and photothermal properties which stem from their localized surface plasmon resonances and result in a wide range of applications in various fields such as catalysis, optical data storage and hyperthermic cancer treatment therapy. The heat-induced deformation behaviour of such nanorods plays a critical role in many applications. So far, most research has been focused on the photothermal stability of uncoated gold nanorods. In the first part of the talk I will discuss the effects of a mesoporous silica coating on the deformation behaviour of Au NRs, since such a coating is known to enhance the thermal and colloidal stability of nanoparticles. I studied the deformation of silica-coated gold nanorods on a single particle level under femtosecond illumination, analysing both shape and plasmon resonances by transmission electron microscopy and electron energy loss spectroscopy. During the second part of my talk I will show that the enhanced thermal stability due to a silica-coating can be exploited to make anisotropic alloys. I will present recent in-situ heating measurements of bimetallic core-shell NRs showing that such a coating can protect the NR shape during alloying. I will furthermore show that alloying results in additional control over optical properties and increased thermal stability.

Abstract:
Core/shell iron oxide nanoparticles have recently received increasing interest because they combine a biocompatible iron oxide (magnetite/maghemite) shell with a high magnetic (iron/wüstite) core. Such core/shell morphologies have been proposed for different biomedical applications, including magnetic resonance imaging, drug delivery and magnetic hyperthermia. Both the shape as well as the oxidation type of iron will have a huge influence on the physical properties. Answering questions on the shape and the oxidation type is not straight forward, since both the core and the shell contain the same chemical elements, making them indistinguishable by conventional techniques such as HAADF-STEM or EDX. We will show that MAADF-STEM tomography and EELS tomography provide us with a solution.
A second study focussed on the visualization of twin planes in 3D. The visualization of structural defects in nanoparticles in 3D can be very challenging. For this purpose, we propose the use of a dose-efficient approach, where LAADF-STEM and HAADF-STEM tilt series are acquired simultaneously. In this way, quantitative information on the shape of the nanoparticles can be obtained together with a clear visualization of the twin planes. Furthermore, the technique is applied for the investigation of the location of a core/shell seed with respect to the twinning planes in nanomaterials.

Abstract:
Perovskite oxides are one of the most abundant natural materials on the Earth. The flexibility of the perovskite structure allows to accommodate cations of different sizes and valences. Wide variety of properties have been demonstrated in bulk perovskites, at their interfaces, or heterostructures made from them including room-temperature ferroelectricity, giant piezoelectricity, quantum oscillation, two-dimensional superconductivity, and many more. The elemental diversity, stability even under off-stoichiometry conditions, and variety of crystal symmetries in the perovskite structure make it a natural host for a range of defects as well. While several point and planar defects have been reported in perovskites with some of them quite unique for perovskites, no new line defect, other than standard dislocations, has been observed until recently. We discover and characterize this new line defect, observed in perovskite NdTiO3 films, using a combination of high-resolution analytical scanning transmission electron microscopy imaging, atomic-scale energy dispersive X-ray spectroscopy and electron energy-loss spectroscopy, and ab initio calculations.

In the last decades researchers started developing the ability to manipulate matter at the nano and atomic scales. The characterization of these materials revealed the influence of size, structure and composition on the peculiar properties exhibited. A fundamental instrument aiding the development of new nanomaterials by enabling the measurement of these properties is the Transmission Electron Microscope (TEM). Conventional TEM only allows for two-dimensional imaging of specimens, often hindering a complete characterization. Combination of TEM and tomography overcomes this limitation, allowing to retrieve a three-dimensional reconstruction of the analyzed sample.
The increasing complexity of synthesized systems though, built in the attempt of achieving particular properties for applications in several fields such as catalysis, signal enhancement or drug delivery, poses new challenges to researchers involved in their characterization. The development of new methods, techniques and instruments is therefore necessary in such occasions in order to obtain a complete description of these samples.
An example of complex systems requiring a challenging characterization is given by nanoparticle assemblies. These structures, created by promoting the self-assembly of hundreds or thousands of nanoparticles, can extend for hundreds of nanometers or even microns, with either an ordered or disordered configuration. Their properties can be tuned by changing the positions of the building blocks, the type of packing and the inter-particles distances. A thorough quantitative characterization is therefore needed to study the relationship between structure and properties, and how changing the former can influence the latter.
Another fundamental problem that has been tackled extensively by several research groups in the recent years, is the determination of the three-dimensional elemental distribution in nanostructures, which can be achieved by combining Energy Dispersive X-ray Spectroscopy (EDXS) and tomography. The recent introduction of multiple detectors systems such as FEI Super-X detector, finally enabled this combination, but earlier attempts, although producing promising results, were still hampered by instrument limitations and lack of proper EDXS quantification methods.
Here, the development of novel techniques and algorithms for electron tomography will be presented. Specifically, it will be shown how quantitative analysis of complex nanoparticles assemblies can be performed by exploiting prior-knowledge in the reconstruction process, and few systems will be examined. Furthermore, another new technique for quantitative EDXS analysis of nanoparticles in 2D and 3D will also be presented. Here, the synergistic combination of quantitative EDXS and HAADF-STEM tomography enables to overcome limitations that previously hindered any reliable quantitative chemical characterization in 3D.

Abstract:
The modern Transmission Electron Microscope (TEM) is a complete analytical tool with only one inconvenience – it has a very small chamber size. The challenges in miniaturizing an entire experimental setup to fit in a 3 mm diameter footprint have been formidable, constraining in-situ TEM studies to mostly real time qualitative visualization. We approach the problem with micro-electro-mechanical systems (MEMS), a perfect foil for the TEM for quantitative testing. We present a setup capable of performing mechanical tests inside a transmission electron microscope (TEM) at elevated temperature up to 1000 °K. The MEMS device is fabricated on silicon-on-insulator (SOI) wafer and has integrated heaters, force sensors and thermal actuators. The device can be co-fabricated with thin films deposited on the wafer, as long as they can be patterned and subsequently released from the substrate to create freestanding uniaxial tension specimens. Young’s modulus, fracture stress as well as stress-strain relationship of thin films at high temperatures can be demonstrated to visualize deformation and fracture mechanisms inside the TEM. The device is demonstrated on metal films as well as graphene, molybdenum di-sulfide and boron nitride. The advantage of the MEMS approach is the ability to multi-function in a single chip. We demonstrate thus by adding microelectrodes for in-situ measurement of thermal and electrical conductivity inside the TEM.

Woensdag 19 juli 2017, Lokaal U.241 (Campus Groenenborger)

Voordracht georganiseerd door TGM

Onderwerp: Applications of the k.p theory to silicon thin layers
Spreker: Prof. Milan Tadic, University of Belgrade and CMT, University of Antwerp

Abstract:
The talk deals with the 30-band k.p theory and its applications to silicon quantum wells. Numerous spurious solutions are found in the energy spectra of those quantum wells and the algorithm to remove them is proposed. The interband optical absorption in the Si/SiO2 quantum well is calculated as a function of the well width W, and the effective direct band gap is found to agree with the 1/W2 scaling result of the single-band model. Finally, the valley splitting in thin silicon layers is investigated. It was shown that it is necessary to restrict the computation basis to the first Brillouin zone to achieve a correct description of the valley splitting by the 30-band model.

Woensdag 21 juni 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
First-principles calculations of the absorption and diffusion of small Fluorine clusters on graphene were performed by using Density Functional Theory (DFT). Several different absorption congurations of fluorine dimers were considered on one side of the graphene sheet (cis-clusters) or at both sides (trans-clusters). The energetically most favorable absorption configuration for cis-clusters corresponds to the para configuration, while for trans-clusters, the most favorable is the ortho conguration. The energy barriers for the diffusion of Fluorine atoms were also calculated.

Woensdag 14 juni 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
Knowing the exact number of particles N, and taking this knowledge into account, the quantum canonical ensemble imposes a constraint on the occupation number operators. The constraint particularly hampers the systematic calculation of the partition function and any relevant thermodynamic expectation value for arbitrary but fixed N. On the other hand, fixing only the average number of particles, one may remove the above constraint and simply factorize the traces in Fock space into traces over single-particle states. As is well known, that would be the strategy of the grand-canonical ensemble which, however, comes with an additional Lagrange multiplier to impose the average number of particles. The appearance of this multiplier can be avoided by invoking a projection operator that enables a constraint-free computation of the partition function and its derived quantities in the canonical ensemble, at the price of an angular or contour integration.
Introduced in the recent past to handle various issues related to particle-number projected statistics, the projection operator approach proves beneficial to a wide variety of problems in condensed matter physics for which the canonical ensemble offers a natural and appropriate environment. In this light, we present a systematic treatment of the canonical ensemble that embeds the projection operator into the formalism of second quantization while explicitly fixing N, the very number of particles rather than the average. Being applicable to both bosonic and fermionic systems in arbitrary dimensions, transparent integral representations are provided for the partition function Z_N and the Helmholtz free energy F_N as well as for two- and four-point correlation functions. The chemical potential is not a Lagrange multiplier regulating the average particle number but can be extracted from F_{N+1} - F_N, as illustrated for a two-dimensional fermion gas. As a particular application to semiconductor device simulations, we quote the use of the canonical treatment to predict a MOSFET threshold voltage, typically characterizing a regime with very low electron concentrations, for which the grand-canonical description fails.

Abstract:
The lecture will outline the results obtained during the first 2 months of my internship as a master student at EMAT.
The internship is part of a project to design advanced noble metal-free transition 3d-metal (Mn, Fe,Co and Ni) nano-oxides and hydroxides possessing high activity both in the oxygen reduction (ORR) and theoxygen evolution reactions (OER), in view of their application at positive electrodes of unitized regenerative fuel cells with alkalinemembranes.. However, the structure refinement of such materials is necessary for a correct description of their properties. Combining transmission electron microscopy withpowder X-ray diffraction is a powerful method for reaching this goal. The lecture will discuss the results of this combination of techniques on several of the candidate materials.

Woensdag 7 juni 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger

Abstract:
The advance in the semiconductor industry has led to a significant decrease of the characteristic dimensions of the devices in the last decades. Together with the isolation of the first 2D materials, quantum phenomena started to be of great importance, which induced the need for different theories that could properly describe the electronic properties of such structures.
The first part of the talk will be based on the brief review of different theoretical frameworks for electron transport calculations, starting from macroscopic semiclassical Boltzmann theory, to quantum theories such as Kubo linear response and Landauer-Büttiker scattering formalism. We’ll try to emphasize not only their domains of validity and limitations but also their connection.
After the introductory part, we will focus on the numerical effort needed for calculation of transport properties using Kubo formulas in the linear response regime, which is suitable when one is interested in bulk properties of different materials. We will show the application of the method to different structures in the presence of various fields and disorder realisations. Remarkable properties such as preservation of chiral symmetry in the presence of vacancy disorder will be shown in graphene, and it’s breaking in twisted bilayer graphene.

Abstract:
Thanks to their unique properties and numerous applications in a wide range of materials and devices, nanoparticles have attracted enormous attention from the scientific and industrial community during the last years. Therefore, in order to deeply understand their properties, a detailed structural characterization and chemical mapping at atomic level are required. In this talk, we will discuss about a method that allows us to quantify the theoretical limits with which atomic columns or atoms of a nanocluster can be located in 2D and 3D, respectively, from a scanning transmission electron microscopy experiment. The concept of this study is explored from a theoretical point of view. Therefore, the combination of both statistical parameter estimation and image simulations is proposed.

Vrijdag 19 mei 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Abstract:
The talk discusses different STEM techniques for quantitative measurements of strain or electric fields. Discussions of the different techniques are each time followed by an example of application to a material system.
In the first part of the presentation three different STEM techniques for strain measurements with increasing strain precision and decreasing spatial resolution are shown: atom position determination using multi-reference rigid-registration (Zorro) and template-matching (TeMA), strain from two-dimensional scanning moiré patterns even for non-square crystal geometry, and nano-beam precession diffraction for strain mapping.
The second part of the talk is discussing the measurement of (mesoscopic) electric fields in STEM. First, DPC is assessed for different convergence angle settings and compared to (off-axis) holography as a more established CTEM based technique. Artifacts in DPC from dynamical diffraction are discussed in terms of convergence angle and sample tilt and also experimentally evidenced in a strained sample by comparison with holography. Afterwards, a new technique to measure electric fields by means of nano-beam precession diffraction is demonstrated. This technique that allows to determine electric fields and strain from the same data set is used to map piezo-electric fields connected to the strain fields of individual dislocations.

Donderdag 18 mei 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
I will give an overview of topological band theory through some simple two-band model systems. I will start from diatomic molecules and the Su-Schrieffer-Heeger model for polyacetylene and extrapolate the physics of these models to graphene and 3D Weyl semimetals.

Abstract:
For future lead and lead-bismuth cooled fast reactors, stabilized austenitic stainless steels are being considered as fuel cladding material. Materials in the core of such reactors are subject to high temperatures (300-500°C) and high fluxes of fast neutrons. These conditions give rise to the formation of radiation defects, which when accumulated can jeopardize the geometrical stability and mechanical properties of the material. Radiation defects arise from clustering point defects, so mitigation strategies for increasing the lifetime of components in the reactor include optimizing material composition and thermo-mechanical treatments.
Cold worked (15-25%) 15-15Ti steels (steel with 15 wt% of both Ni and Cr and about 0.4 wt% of Ti) are alloys of particular interest for their demonstrated improved radiation resistance as compared to commercial steels. Through heat treatment as well as under certain conditions in the reactor, TiC nanoprecipitates of size 1-10 nm nucleate on dislocations, as well as on twin and grain boundaries. It is believed these semi-coherent precipitates act as point defect traps, which can improve radiation resistance.
This presentation will focus on the efforts that have been made so far to map out the effect of heat treatment on microstructure, with emphasis on the TiC nanoprecipitates and the dislocation structure. TEM DF imaging and BF Moiré fringes (together with CBED and EELS to measure thickness) were used to estimate their size and number density. Dislocation structures were studied with WBDF. EFTEM imaging confirmed the presence of Ti and C in the particles. Mismatch dislocations were directly imaged by HR-STEM. Finally, an outlook is presented on very recent work involving ion irradiation experiments.

Woensdag 10 mei 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
In this talk, after a basic review of molecular dynamics (MD) simulations, I will present our recent numerical/experimental study of graphene nanobubbles in collaboration with School of Chemical Engineering and Analytical Science (the Manchester university). This work is accepted and will appear in Nature Communications.
Van der Waals (vdW) interaction between two-dimensional crystals (2D) can trap substances in high pressurized (of order 1 GPa) on nanobubbles. Increasing the adhesion between the 2D crystals further enhances the pressure and can lead to a phase transition of the trapped material. We found that the shape of the nanobubble can depend critically on the properties of the trapped substance. In the absence of any residual strain in the top 2D crystal, flat nanobubbles can be formed by trapped long hydrocarbons (i.e. hexadecane). For large nanobubbles with radius 130 nm, our atomic force microscopy measurements show nanobubbles filled with hydrocarbons (water) have a cylindrical symmetry (asymmetric) shape which is in good agreement with our molecular dynamics simulations. This study provides insights into the effects of the specific material and the vdW pressure on the microscopic details of graphene bubbles.

Vrijdag 5 mei 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Abstract:
MAX phases are nanolaminated carbides or nitrides, which show both metallic properties such as good thermal/electrical conductivity, thermal shock resistance, good machinability, damage tolerance and ceramic properties like good mechanical properties at high temperature, high oxidation and corrosion resistance. Among the MAX phase family, Ti-Al-C based phases have been extensively studied in literature and one of their outstanding properties is their oxidation resistance due to the formation of protective Al2O3 layers. Zr-Al-C based MAX phases: Zr3AlC2 and Zr2AlC are some of the new members of the expanding MAX phase family and their features are yet to be explored. Their particular importance lies in the lower neutron absorption cross-section of Zr, making these MAX phases especially promising for future nuclear applications. To satisfy both the requirements of next generation (GENIII+) nuclear reactor designs and the safety requirements to prevent material failure accidents, new accident tolerant cladding materials should be developed. These materials should have high resistance to oxidation and corrosion and should withstand high temperatures and radiation doses for prolonged exposures. One of the methods to fine-tune the properties of MAX phases is by creating solid solutions with different M, A or X element. Studies have shown that solid solutions can have better properties than their end members. The focus of this PhD to date was on the synthesis and characterization of solid solution MAX phases in the (Zr,Ti)-Al-C system. Further work will focus on improving the phase purity of the obtained ceramics and studying their irradiation, oxidation resistance, coolant interactions and mechanical properties.
Solid solution MAX phases (Zr1-xTix)3AlC2 (312) and (Zr1-xTix)2AlC (211) with x changing from 0 to 1 were synthesized by reactive hot pressing of ZrH2, TiH2, Al and C powders between 1350 and 1700°C. The produced ceramics contained large fractions of 211 and 312 MAX phases, while strong evidence of a 413 stacking was also found. Moreover, (Zr,Ti)C, ZrAl2, ZrAl3, and Zr2Al3 were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr0.33,Ti0.67)3Al1.2C1.6 stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed M6X octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase.

Donderdag 4 mei 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
In recent years, the experimental realization of LandauZener-Stuckelberg (LZS) interferometry in several systems has emerged as a tool to study quantum coherence under strong driving. In two-level systems driven by an ac force, the accumulated phase between periodically repeated LZS tunneling events at avoided crossings gives place to constructive or destructive interferences, depending on the driving amplitude and the detuning from avoided crossing.
I will discussLZS interferometry in superconducting flux qubits coupled to an Ohmic quantum bath. We have found a dynamic transition manifested by a symmetry change in the structure of the LZS interference pattern, plotted as a function of ac amplitude and dc detuning. The dynamic transition is from an LZS pattern with nearly symmetric multiphoton resonances to antisymmetric multiphoton resonances at long times (above the relaxation time). I will also discuss the effect of the quantum bath spectral structure and the system-bath coupling on the LZS interference pattern and compare with experiments in flux qubits.

Vrijdag 28 april 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Abstract:
In today’s world, the field of nanotechnology is evolving rapidly, leading to more sophisticated products with improved properties. In this field, advanced material’s characterisation techniques are required since the exact atomic structure of materials determines their physical and chemical properties. A popular method for this task is the transmission electron microscope (TEM). In this presentation, different imaging modes of the TEM are quantitatively evaluated in order to help the microscopy user in deciding which technique is most suitable for his or her purpose.
In the first part, different TEM modes which are capable of visualising light elements are quantitatively compared in terms of accuracy and precision by using statistical parameter estimation theory. Furthermore, the effect of post-processing techniques on scanning TEM (STEM) images is examined. In order to provide an outlook to the future, simulated images, in which the unavoidable presence of Poisson noise is taken into account, are used to determine the ultimate precision.
When visualising atomic structures, the contrast in high angle annular dark field (HAADF) STEM images is sensitive to both thickness and chemical composition. However, when differences in atomic number are negligible or both thickness and chemical information need to be extracted from a single viewing direction, the information given by HAADF STEM images becomes insufficient. Therefore, spectral imaging by using energy dispersive X-rays (EDX) is often essential. In this presentation, the possibility to use these spectral images for atom counting will be explored.
Finally, the opportunities that new direct electron detectors can offer are investigated by using advanced statistical methods. It will be shown how chemical information is distributed in convergent beam electron diffraction (CBED) patterns. This information is crucial for imaging nanomaterials under optimal experimental conditions in the currently available STEM detectors and the ordered direct electron detectors by EMAT.

Vrijdag 21 april 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Voordracht georganiseerd door EMAT

Onderwerp : Measuring band gaps of wide-bandgap semiconductors including diamond with STEM-EELS
Spreker: Svetlana Korneychuk, EMAT, University of Antwerp

Abstract:
Manufactured semiconductor devices generally consist of many layers with different electronic and optical properties and the knowledge about the bandgap of each layer and energy band alignment is very important in order to understand and improve the performance of the device. The conventional methods of band gap measurement, even though they’re well developed and accurate, do not give band gap information at the local scale.
Low loss electron energy loss spectroscopy (EELS) in transmission electron microscopy (TEM) can retrieve the information about dielectric properties of the material including the band gap. However, ambiguity of data interpretation didn’t allow EELS to become a conventional technique for band gap measurements. The retardation losses, including e.g. Cherenkov radiation emission, have an undesirable impact on the low loss signal complicating the retrieval of the band gap signal. Nevertheless, several attempts to overcome the unwanted effect of retardation losses have been shown in EELS community.
In this work we demonstrate a technique to measure the band gap with EELS and highly reduce the influence of unwanted effects. All undesirable losses take place in a certain range of angles which can be shown by simulations using the so-called Kröger formula. This equation allows to calculate the energy losses of fast electrons in thin foils with retardation starting from the dielectric properties. We minimize the impact of undesirable losses by selecting the retardation-free part of the outcoming signal using an annular (Bessel) aperture. A decrease of Cherenkov effect is also achieved by operating at 60 kV - the minimal possible voltage in our microscope. The obtained band gap values are in good agreement with the conventional measurements, with a much improved spatial resolution.

Vrijdag 31 maart 2017 , 11.30 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
Atomic clusters and assemblies of nanoparticles have attracted increasing scientific interest during the last years, thanks to their unique properties and numerous applications in a plethora of scientific fields. It has been proven experimentally that the structure of such nanomaterials is inseparably connected to their characteristic properties. Therefore, in order to deeply understand the structure-to-property relationship, a detailed structural characterization is of utmost importance.
In the first part of my talk, I will show that by the use of advanced Transmission Electron Microscopy (TEM) techniques in combination with X-ray diffraction (XRD) data and a careful image analysis, one is able to unravel the structure of luminescent Ag clusters confined in Faujasite zeolites. Furthermore, approaches for the characterization of free standing atomic clusters will be presented.
The second part of my talk will be dedicated to the results of the structural and morphological characterization of assemblies of nanoparticles with varying shape, size and chemical composition. When investigating such complex structures, the information provided by (S)TEM images is not adequate since they only correspond to 2D projections of a 3D object and very often can be very misleading. For a more reliable structural and morphological characterization in 3D, electron tomography should be used. Since the turn of the century, the technique has been used to investigate the 3D structure of materials at the nanometer scale and below. Furthermore, the need for the optimization of both the acquisition and reconstruction processes will be discussed, in cases where more complex or bigger assemblies are investigated.

Woensdag 29 maart 2017, 16.00 u., Lokaal U.241 (Campus Groenenborger)

Abstract:
We study the electronic and structural properties of excitons and trions in 2D transition metal dichalcogenides using two different methods: i) A few-body model in which we construct the Hamiltonian in the basis formed by all the possible products of single-particle states. We decouple the corresponding stationary Schrödinger equation and solve the resulting differential equation self-consistently. ii) The stochastic variational method (SVM) in which a variational wave function is used which is expanded in a basis of a large number of correlated Gaussians. We find good agreement between the results of both methods as well as with other theoretical works. However, the few-body model is a multi band model whereas the SVM is a single band model. This leads to differences between the two methods when (interband) interactions are strong.

Vrijdag 24 maart 2017, 11.30 u., Lokaal U.244 (Campus Groenenborger)

Abstract:
The effect of room temperature aging combined with low temperature cycling on the structure of Ni-Ti and martensitic transformation is studied. DSC measurements show that Ms decreases with time, and it decreases even faster when aging is combined with DSC cycling, indicating some microstructural changes in the material. HRTEM and HAADF STEM imaging does not reveal any change in the material structure, however, strong localized and periodic diffuse intensities are observed in electron diffraction patterns. Applying Cluster Model which ascribes the shape and periodicity of the diffuse intensity to that of short ranged microdomains reveals the formation of pure Ni columns along [111]B2 crystallographic directions in the cubic Ni-Ti lattice which can be considered as early stages of precipitates in the low temperature aged samples. Comparing the diffuse intensities of different samples shows enhancement of short-range ordering in system with DSC cycling and aging in the thermomechanical history.

Abstract:
The mechanical design of components depends on the mechanical properties of such materials. At the macro-scale, the mechanical properties can be obtained from well-defined mechanical testing methods. However, at the micro/nano-scale, for which there is a dramatic demand of smaller and more durable micro-electro-mechanical systems (MEMS) and electronics, the development of micro/nano-mechanical testing instruments has recently enabled the investigation of the mechanical properties at such scales. In the present work the emphasis is on fatigue as one of the main failure engineering modes of materials. In practice, compressive and tensile fatigue properties of small scale materials are investigated and correlated to the observed mechanical properties and the dislocation structure evolution and mechanisms.
An in-situ SEM micro/nano-compression test was used to investigate the compressive fatigue response of single crystal and bi-crystal Nickel micropillars and their relevant dislocation microstructure were studied by conventional and advanced ex-situ TEM techniques. It has been observed that dislocation propagation and dislocation interaction are the main governing mechanisms in such small-scale samples.
In-situ TEM tensile testing was also used to investigate the tensile fatigue response and to observe directly the governing mechanisms in small-scale samples. The first step, and the main challenge, was to prepare a proper sample, not-affected and not-damaged by used sample preparation methods. Different methods like combination of twin-jet electropolishing, FIB and in-situ TEM have been used to reach this goal. Tensile fatigue tests were carried out on the defect-free Nickel single crystal samples. It was observed that in such scale the governing mechanism is a combination of dislocation propagation and dislocation starvation.

Vrijdag 10 maart 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Abstract:
Nowadays neural networks remarkably improved the quality of state-of-the-art computer vision, image denoising, semantic segmentation, natural language processing, speech recognition and other techniques. Most of these improvements were realised by using convolutional neural networks (CNN) which is inspired by the organization of the brain visual cortex and dates back decades. Despite the explosive popularity of CNN in different fields, their applications in the field of transmission electron microscopy (TEM) is very limited. In this talk, I will show how CNN can help us to crack several difficult TEM problems including atomic column identification, atom counting, location of impurity atoms, tomography alignment, real-time compressed sensing, and automatic element identification in electron energy loss spectroscopy.

Vrijdag 3 maart 2017, 11.30 u., Lokaal U.408 (Campus Groenenborger)

Abstract:
Plasmonics, the science and technology of the interaction of light with metallic objects, is fundamentally changing the way we can detect, generate and manipulate light at the nanoscale. While the field is progressing swiftly thanks to the availability of nanoscale manufacturing and analysis methods, fundamental properties such as the symmetries of the plasmonic excitations cannot be accessed by direct measurements, leading to a partial and sometimes incorrect understanding of their properties.
In particular, a very important role is that of electron energy loss spectroscopy (EELS), the only method that currently allows to map at the nanoscale the intense electric fields that characterise plasmonic resonances. One of its main drawback is that, while it is sensitive to the field's intensity, it can't detect its orientation, hiding their real symmetry and shape.
We have overcome this limitation by deliberately modifying the wave–function of the electron beam to match the symmetry of the plasmonic excitations. After briefly introducing electron beams phas manipulation methods, I will show experimentally and theoretically that this new approach allows the selective detection of specific plasmon modes within metallic nanoparticles while filtering out modes with other symmetries.
This method shows some resemblance to the widespread use of polarised light for the selective excitation of plasmon modes but adds the advantage of locally probing the response of individual plasmonic objects and a far wider range of symmetry selection criteria.

Abstract:
In this talk, I will show a user-friendly software package, called StatSTEM, which has been released in the summer of 2016. StatSTEM includes well-established image quantification methods using model-based fitting. The advantages of using this model-based approach will be illustrated.
Furthermore, I will discuss a method to create three-dimensional atomic models from a single Z-contrast image. Our new method will be validated against state-of-the-art compressive sensing electron tomography. This new approach allows for the characterization of beam-sensitive materials, or where the acquisition of a tilt series is impossible. As an example, the utility of this alternative approach is illustrated by the 3D characterization, at the atomic scale, of a nanodumbbell on an in situ heating holder.

Abstract:
he theory of micromagnetism, in which the magnetisation is described by a continuous vector field, describes ferromagnetism on the submicron-scale. In this talk, I will give an introduction to spin waves in ferromagnetic films using the micromagnetic framework. Starting from the Landau-Lifshitz-Gilbert equation, which governs the dynamics of the magnetization, it is fairly straightforward to calculate the spin wave dispersion relation of ferromagnetic films. I will discuss how the dispersion relation depends on the exchange interaction strength, the presence of uniaxial anisotropy, and the chiral Dzyaloshinskii-Moriya interaction strength. In the second part of my talk, I will show how one can efficiently compute the eigenmodes of spin structures such as vortices and skyrmions.

Abstract:
In optical tomography light does not behave in the ideal way that X-rays can do in X-ray computed tomography. Effects of diffraction, refraction, and scattering can severely affect the image quality in optical tomography when not properly taken into account. In this presentation I will show how these effects can be mitigated by using physical measurement principles and computational image reconstructions algorithms. Results on tomographic reconstructions of zebrafish demonstrate the opportunities this offers for high resolution imaging of small animals.

Abstract:
Our inverse problem of interest is the retrieval, in two or three dimensions, of an object from a series of (S)TEM images. Some instances with linear image formation are considered, but also cases with multiple scattering. The inclusion of prior knowledge through compressed sensing is treated as well.

Abstract:
The way the crystal structure of a cathode material changes during operation of Li-ion battery has a crucial impact on its performance. Recently, new polymorph of Li2FePO4F was obtained, which can be considered as a promising cathode material for Li-ion batteries. By electrochemical ion-exchange it was prepared from LiNaFePO4F, which has a 3D framework structure formed by FeO4F2 octahedra and PO4 tetrahedra. Na resides in the channels of framework and it can reversibly (de)intercalated. We used initial LiNaFePO4F as a cathode in the electrochemical cell and charged it at elevated temperature. As a result Na was fully removed from the channels. Upon discharge of the battery Li was introduced in the framework forming Li2FePO4F.
In this work we determined the crystal structure of Li2FePO4F by means of electron diffraction tomography. Oppositely to our expectations, only 70% of Li occupy former positions of Na in the channels. The remaining 30% Li was found in the Fe position, showing the presence of Li-Fe anti-site defects. To find out the origin of such disorder, we determined the crystal structure of LiFePO4F and Li2FePO4F prepared at room temperature. The anti-site defects were found in both cases, while the initial one does not have them. We also studied a compound with similar composition, LiFePO4. However, after cycling even at elevated temperature such dramatic anti-site disorder as for Li2FePO4F was not found. Therefore, we suppose that the main reason is a specific feature in the crystal structure that is different for Li2FePO4F and LiFePO4: in Li2FePO4F case two oxygens are coordinated by one P and three Li atoms, and two out of these three Li leave structure upon charge. As a result, in charged state these O atoms become underbonded which cannot be compensated by shrinking of the P-O bonds. Therefore, Fe3+ atoms partially migrate into the Li position to eliminate the misbalance. In LiFePO4all O atoms are connected with electrochemically active Li and Fe and thus no such compensation is needed.

Abstract:
Aberration-corrected (Scanning) transmission electron microscopy ((S)TEM) is a very powerful technique to characterize the structure and the composition of materials at the atomic scale. However, one should never forget that (S)TEM images are only two-dimensional (2D) projections of a three-dimensional (3D) object. By using electron tomography one is able to determine the shape and structure of an object in 3D. In this talk, different approaches combined with electron tomography are proposed to recover the structure and composition of nanostructures at the atomic scale.
In the first part of the talk, an approach is proposed where a combination of X-ray energy dispersive spectroscopy (XEDS) in an aberration-corrected scanning transmission electron microscope with electron tomography, which enables a 3D chemical mapping at the atomic scale.
In the second part of the talk, a different approach is discussed where high angle annular dark-field scanning TEM (HAADF-STEM) is combined with exit wave reconstruction (EWR). When using high resolution TEM (HR-TEM) image interpretation and quantification is complex due to the strong radiation-matter interaction and the sample’s exit wave is modified by the microscope. Consequently the contrast observed in the images is an interference effect and is not directly interpretable. By using EWR one is able to retrieve the amplitude and phase of the exit wave from the object. By combining the phase images from different zone axes with HAADF-STEM tomography and by using a compressive sensing based reconstruction algorithm, it is possible to obtain a tomographic reconstruction at the atomic scale.
In the last part of the talk, the combination of low dose imaging with the three-dimensional EWR will be discussed. Such an approach can enable the structural characterization of beam sensitive materials at the atomic scale.

Abstract:
Recently, two dimensional materials with noncentrosymmetric structure have received significant interest due to their potential usage in piezoelectric applications. It has been reported by first principles calculations that relaxed-ion piezoelectric strain (d11) and stress (e11) coefficients of some transition metal dichalcogenide monolayers are comparable or even better than that of conventional bulk piezoelectric materials [1]. Furthermore, piezoelectric coefficient of MoS2 has been measured as 2.9×10−10 C/m[2] , which agrees well with the mentioned theoretical calculation. Afterwards, this exceptional potential has been deeply investigated by the calculation of the piezoelectric properties of various single layer structures: two dimensional transition metal dichalcogenides [3] , transition metal oxides [3] , group II oxides [4] , and hexagonal group III-V [4] , IV-VI [5] and II-VI [6] compounds. The reported results have clearly shown that not only the Mo- and W-based transition metal dichalcogenides but also the other materials with Cr, Ti, Zr and Sn exhibit highly promising piezoelectric properties. Moreover, d11 coefficient of some IV-VI and II-VI (see Figure 1) compounds have been predicted as quite larger than that of transition metal dichalcogenides and the bulk materials, α-quartz, w-GaN, and w-AlN which are widely used in current applications. In conclusion, the reported first principle predictions clearly reveal that monolayer semiconductors are strong candidates for future atomically thin piezoelectric applications such as transducers, sensors, and energy harvesting devices.

Abstract:
During my Ph.D. research, I have investigated the structure of specific perovskite based oxides in order to establish the correlation between the structure and magnetic properties. The final goal of our study is to design new compounds with a potential for applicable properties, for instance relaxor ferromagnetism.
The crystal structures have been solved using a combination of transmission electron microscopy techniques, including selected area electron diffraction (SAED) combined with real space imaging using different techniques such as high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), annular bright field scanning transmission electron microscopy (ABF-STEM) and energy dispersive X-ray spectroscopy-STEM (EDX-STEM). Based on these studies, models have been proposed and refined for different triple perovskite compounds. With these models we have tried to explain the variations in the physical properties of the samples and we have compared them to similar perovskites. The disclosed relations have rendered fundamental knowledge, applicable for the optimization of the properties of the investigated materials as well as of related perovskite materials.

Onderwerp: Different structure models of group IV element monolayer
Spreker: Linyang Li, CMT, University of Antwerp

Abstract:
Since monolayer graphene was successfully realized in 2004, its unusual Dirac cone band structure has attracted great attention. Its honeycomb lattice with perfect hexagonal symmetry plays a crucial role in the formation of the Dirac cone with linear dispersion. Similarly, other group IV element Si (Ge/Sn) monolayer with a hexagonal honeycomb lattice was proposed, called as silicene (germanene/stanene). In experiment, they were successfully synthesized on different substrates. Besides the hexagon structure, another stable structure model, dumbbell structure, was applied on the element Si (Ge/Sn) monolayer, which exhibits a lower energy than that of the hexagon structure.
In this talk, Linyang Li will compare the structure, energy and band structure of the silicene with dumbbell and hexagon structures and some our new results on dumbbell structure will be shown.

Abstract:
We report an innovative method to quantitatively optimise the experiment design for discrete estimation problems in high resolution (scanning) transmission electron microscopy (HR(S)TEM). In a first part of this talk, this quantitative approach is used to investigate the optimal experimental settings for both detecting and locating light elements in HR(S)TEM images. The principles of detection theory are then used to quantify the probability of error as an optimality criterion for the detection of light atoms. To determine the optimal experiment design for locating light atoms, use is made of the so-called Cramér-Rao Lower Bound (CRLB). It is investigated if a single optimal design can be found for both the detection and locating problem of light atoms.
In a second part of this talk, our quantitative approach is also used to optimise the experiment design for nanoparticle atom-counting from both TEM and STEM images. So far, HRSTEM has been shown to be an appropriate method to count the number of atoms in a projected atom column. Recently however, it has been shown that one HRTEM image using negative spherical aberration imaging suffices to count atoms as well. Our quantitative approach based on the principles of detection theory is used, in order to determine the limits to the precision with which the number of atoms in a projected atom column can be estimated. The capabilities of both imaging techniques, HRSTEM and HRTEM, are investigated and compared in terms of atom-counting reliability.
In the last part of this talk, an experimental application is shown where the domain wall in a LiNbO3 crystal is quantified using the principles of statistical parameter estimation theory.